1. Field of the Invention
The invention relates to a method for evaluating a state of a soot sensor in a motor vehicle having an internal combustion engine. The loading of an exhaust gas stream of the internal combustion engine with soot particles is detected with the soot sensor that is electrically connected to an evaluation circuit permanently installed in the motor vehicle in evaluation circuit detects and evaluates conductivity between at least two measuring electrodes of the soot sensor. The invention also relates to a soot sensor which is operated with this method.
2. Description of the Related Art
The enrichment of the atmosphere with pollutants from exhaust gases is currently being hotly debated. Associated with this is the fact that the availability of fossil fuels is limited. In reaction to this combustion processes in the internal combustion engines are thermodynamically optimized so that their efficiency is improved. In the field of motor vehicles, this has resulted in the increasing use of diesel engines. However, the disadvantage of this combustion technology over optimized spark ignition engines is a significantly increased emission of soot. Soot is extremely carcinogenic, particularly due to the precipitation of polycyclic aromatics, which have already been the subject of various regulations. For example, exhaust gas emission standards with maximum limits for soot emissions have been issued. It is therefore necessary to specify cost-effective sensors which reliably measure the soot content in the exhaust gas stream of motor vehicles.
The use of such soot sensors serves to measure the currently emitted soot so that the engine management system in an automobile in an ongoing driving situation is provided with information to be able to reduce the emission values by control adaptations. Furthermore, by using soot sensors it is possible to initiate active exhaust gas purification by exhaust gas soot filters or exhaust gas recirculation to the internal combustion engine. In the case of soot filtering, regenerative filters are used which filter out a significant part of the soot content from the exhaust gas. Soot sensors are needed to detect soot so that the function of the soot filters can be monitored and/or their regeneration cycles can be controlled.
For this purpose, the soot filter, which is also referred to as a diesel particle filter, has a soot sensor connected upstream, and/or a soot sensor connected downstream.
The sensor which is connected upstream of the diesel particle filter serves to increase the system reliability and to ensure operation of the diesel particle filter under optimum conditions. Since this depends to a high degree on the mass of soot which has accumulated in the diesel particle filter, precise measurement of the particle concentration upstream of the diesel particle filter system, in particular the determination of a high particle concentration upstream of the diesel particle filter, is highly significant.
A sensor which is connected downstream of the diesel particle filter provides the possibility of performing on-board diagnostics in the vehicle, and also serves to ensure correct operation of the exhaust gas post-treatment system.
In the prior art there have been various approaches to the detection of soot. One approach which has been pursued to a large extent in laboratories is to use the scattering of light by the soot particles. This procedure is suitable for complex measuring devices. If an attempt is made also to use this procedure as a mobile sensor system in exhaust gas, it is necessary to take account of the fact that approaches for implementing an optical sensor in a motor vehicle are associated with high costs. Furthermore, there are unsolved problems relating to contamination of the required optical windows by combustion exhaust gases.
At present, particle sensors for conductive particles are known in which two or more metallic electrodes, which have electrodes that engage one in the other in the manner of a comb, are provided. Soot particles which are deposited on these sensor structures short-circuit the electrodes and therefore change the impedance of the electrode structure. As the particle concentration on the surface of the sensor rises, it is possible in this way to measure a decreasing resistance or an increasing current when a constant voltage is applied between the electrodes. Such a soot sensor is disclosed, for example, in DE 10 2004 028 997 A1.
However, the measured values which are detected by the soot sensor can change over the service life of the soot sensor. This may take place through deposition of ash on the measuring electrode structure or through a changed temperature cross-sensitivity of the soot. Evaluation of the state of the soot sensor during its entire service life is therefore necessary since without this state evaluation increasingly falsified measured values would result.
EP 1 624 166 B1 discloses a method for assessing the state of a particle sensor in which the loading of an exhaust gas stream with soot particles is detected, in which method the conductivity between two electrodes of a particle sensor which is subjected to the exhaust gas stream is detected. The particle sensor is, for this purpose, temporarily heated and the soot particles which are deposited on the particle sensor are burnt. During the heating, the conductivity between the electrodes of the particle sensor is detected and used for the evaluation of the state of the particle sensor. However, this often leads to highly fluctuating measured values since the electrical conductivity of the soot sensor is a second degree tensor (dyade), that is to say a highly complex multi-dimensional value.